This tutorial describes how you can use PRAAT to perform Multi Dimensional Scaling (MDS) analysis.

MDS helps us to represent dissimilarities between objects as distances in a Euclidean space. In effect, the more dissimilar two objects are, the larger the distance between the objects in the Euclidean space should be. The data types in PRAAT that incorporate these notions are Dissimilarity, Distance and Configuration.

In essence, an MDS-analysis is performed when you select a Dissimilarity object and choose one of the To Configuration (xxx)... commands to obtain a Configuration object. In the above, method (xxx) represents on of the possible multidimensional scaling models.

MDS-analysis

Let us first create a Dissimilarity object. You can for example create a Dissimilarity object from a file. Here we will the use the Dissimilarity object from the letter R example. We have chosen the default value (32.5) for the (uniform) noise range. Note that this may result in substantial distortions between the dissimilarities and the distances.

Now you can do the following, for example:

Select the Dissimilarity and choose To Configuration (monotone mds)..., and you perform a kruskal-like multidimensional scaling which results in a new Configuration object. (This Configuration could subsequently be used as the starting Configuration for a new MDS-analysis!).

Select the Configuration and choose Draw... and the following picture might result.

When you enter noiseRange = 0 in the form for the letter R, perfect reconstruction is possible. The Shepard diagram then will show a perfectly smooth monotonically increasing function.

Weighing the dissimilarities

When you can't have equal confidence in all the number in the Dissimilarity object, you can give different weights to these numbers by associating a Weight object with the Dissimilarity object. An easy way to do this is to select the Dissimilarity object and first choose To Weight. Then you might change the individual weights in the Weight object with the Set value... command (remember: make wij = wji).

Using a start Configuration

You could also use a Configuration object as a starting configuration in the minimization process. Let's assume that you are not satisfied with the stress value from the Configuration object that you obtained in the previous analysis. You can than use this Configuration object as a starting point for further analysis: